Modern fishing fleets commonly undertake cruises over distances of thousands of miles and durations of several months. Such long duration voyages are made possible by refrigerated fish tanks which have the capability of freezing a large catch of fish to minimize spoilage of the fish.
The refrigerated fish tank commonly used in large fishing vessels has remained virtually unchanged over the last thirty years. For example, the fish tanks on board an average (e.g.,1200 ton), modern tuna fishing vessel, would have a capacity of 60 tons of tuna each. Each tank has a generally cubical configuration with a hatch opening at the top for loading the fish. The overhead, deck, fore, aft, inboard, and outboard bulkheads are lined with refrigeration pipe coils. The fish tank employs a pumping system comprising a circulation pump located behind the inboard bulkhead, a suction pipe located near the bottom of the fish tank and covered by a perforated metal guard, and a discharge pipe located in the hatch coaming at the top of the fish tank.
When a fishing vessel has reached a suitable fishing location, sea water is first pumped into the fish tank. The sea water is circulated by the pump and cooled by contact with the refrigeration coils. Catches of fish are then added to the cold sea water in the tank until the tank is full. Using sea water as a cooling medium in this manner the fish may be cooled to approximately 30.degree. fahrenheit. After the fish have been cooled to approximately 30.degree. fahrenheit, the fish tank is drained of sea water and the sea water is replaced by a high salt content brine. The brine is circulated through the fish by the pumping system, and the temperature of the fish is reduced to a point where the fish are completely frozen, i.e., corresponding to a temperature of 10.degree. fahrenheit or less.
The use of a brine flow refrigeration system in fishing vessel fish tanks, along with the fish spoilage inhibiting effects of such a system, have been known since at least 1955. For example, the use of such a system is described in a report by Lionel Farber entitled "Refrigeration of Tuna and Sardines by Sodium Chloride Brines", published in Food Technology, Volume IX, 1955, No. 3, pages 141-147. In such a system, it is important to minimize the time taken for the fish to freeze from 30.degree. fahrenheit to less than 20.degree. fahrenheit. Above 20.degree. fahrenheit, salt will penetrate the flesh of the fish from the strong salt brine solution, reducing the quality of the fish for processing purposes. In addition, until the fish are cooled below 20.degree. fahrenheit, bacterial and enzymic action are not completely halted.This also reduces fish quality and may cause fish spoilage for some percentage of the catch. Accordingly, the optimum brine flow refrigerated fish tank would cause the fish catch to uniformly be lowered below 20.degree. fahrenheit as rapidly as possible.
The brine refrigerated fish tanks currently in use are extremely inefficient in providing rapid and uniform cooling of the fish mass in the tank for several reasons. One source of inefficiency is the uncertain and non-uniform distribution and direction of the brine flow through the fish in the fish tank. The discharge pipe of the pumping system distributes the brine at a single point at the top of the fish mass located in the fish tank. The brine therefore flows approximately linearly to the suction pipe with the brine having minimal circulation around the refrigeration pipes and corners opposite the suction inlet.
Another source of inefficiency of the standard brine flow system is the interference of the fish themselves with the effectiveness of the refrigeration pipes. The fish will tend to lie horizontally in the fish tank and tightly pack around the horizontal refrigeration pipes, thereby covering a substantial percentage of the pipe surface. This packing of the fish around the refrigeration pipes will reduce the brine flow around the pipes and provide an insulating effect thereby reducing the cooling effect of the pipes and the efficiency of the refrigation system.
Yet another inefficiency in the standard brine flow system in existing fish tanks is the nearly total lack of brine flow throughout a substantial portion of the interior of the fish mass. Tuna, and several other types of fish commonly caught by fishing vessels using brine refrigerated fish tanks, have a shape which enables them to pack together very tightly when placed in the fish tank. With the lack of a positive brine flow path through the interior of the fish mass, the packing of the fish can almost totally prevent brine circulation through large portions of the interior of the fish mass.
The effect of the above-noted inadequacies in the existing brine flow system utilized in fishing vessels is that the freezing of the fish in the fish tank will occur very nonuniformly. More specifically, fish packed around the refrigeration pipes will freeze very rapidly below the critical 20.degree. fahrenheit temperature and will suffer only acceptable salt penetration. Similarly, fish located near the actual path taken by the brine through the fish mass will also cool quite rapidly below the critical 20.degree. fahrenheit temperature. On the other hand, fish located away from the actual brine flow path or packed tightly in the interior of the fish mass, will take substantially longer to cool below 20.degree. fahrenheit and will suffer correspondingly greater salt penetration and bacterial and/or enzymic damage.
Accordingly, it is an objective of the present invention to increase the freezing capability of an existing brine tank refrigeration system thereby reducing the freezing period. It is a further object of the present invention to provide positive brine flow throughout substantially all of the fish mass in a brine refrigerated fish tank.
It is a further object of the present invention to provide positive brine flow around a substantial portion of the refrigeration coils in a brine refrigerated fish tank.
It is a further object of the present invention to provide a system suitable for accomplishing the above objectives by relatively easy and inexpensive modification of existing fishing vessel fish tanks employing brine refrigeration systems.